To investigate the assembly-disassembly kinetics of Microtubules (MTs) reassembled to steady state from outer doublet and bovine brain tubulins. We have determined the rate constants at opposite MT ends and have shown that MT assembly-disassembly is not a linear function of the free dimer concentration. We propose to investigate the basis for this unexpected non-linearity. We consider three possibilities. 1) The unexpected kinetics result from a MAP equilibrium with the MT surface, 2) That tubulin oligomers as well as tubulin dimers participate in the MT assembly reaction, 3) That MT ends possses a limited number of assembly sites which become saturated at higher tubulin concentrations, thereby giving rise to Henri-Michaelis-Menten type kinetics.